The model part of the MVC code for a web page that plays tic tac toe.

Tictactoe.js

  const lines = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6],
  ];
  const slotStatus = { empty:0, cross:1, circle:2};
  function display(val){
    switch (val){
      case slotStatus.empty: return "";
      case slotStatus.circle: return "O";
      case slotStatus.cross: return "X";
      default: return "X";
    }
  }
  function is_player_going_to_win ( player, board){
  
    lines.findIndex((X)=>
    {
      let count = 0;
     X.forEach((x)=>{
               if (board[x]===player) ++count;
               else if(board[x] !== slotStatus.empty) --count
     });
        return count===2;
    });
  }
  function has_player_won ( player, board){
  
    lines.findIndex((X)=> 
      X.findIndex((x)=>board[x]!==player) === -1);
  }
  
  function fill_corner( slot,  board){
    let corners=[0,2,6,8];
    //console.log("fill_corner ",slot )
    //TODO use corners.find
    let corner = corners.findIndex((x)=> board[x]===slotStatus.empty);
    if (corner>=0)
       board[corner] = slot;
    return corner>=0;
  }
  function fill_any( player,   board)
  {
    //ToDo: use board.findIndex
    let i = board.findIndex((x)=>board[i]===slotStatus.empty);
    if(i>=0)
        board[i] = player;
  
  }
  function fill_line ( k,  player,   board){
   let i = lines[k].find((x)=>board[x] === slotStatus.empty);
      if(i !== undefined){
        board[i] = player;
      }
  }
  function forced_play( player,  opponent,   board)
  {
  
    let k = is_player_going_to_win(player,board);
    if (k >=0){
      fill_line(k,player,board);
      return true;
    }
    k = is_player_going_to_win(opponent,board);
    if (k >=0){
      fill_line(k,player,board);
      return true;
    }
    return false;
  }
  function play_turn(    board){
    let turn=0;
    board.forEach((x)=> {if (x !==slotStatus.empty)
    {
      //console.log("x=", x);
      ++turn;
    }
    });
    //console.log("turn=", turn)
    switch (turn){
      case 0: 
        board[4]=slotStatus.circle;
        break;
      case 1:
        if (board[4] === slotStatus.empty) {
          board[4] = slotStatus.cross;
        } else {
          board[0] = slotStatus.cross;
        }
        break;
      case 2:
        {
          let middle = [1, 3, 5, 7];
          let diagnol = [8, 8, 0, 0];
          for (let i = 0; i < 4; ++i) {
            if (board[middle[i]] !== slotStatus.empty) {
              board[diagnol[i]] = slotStatus.circle;
              return;
            }
          }
          fill_corner(slotStatus.circle, board);
          break;
        }
      case 3:
      case 5:
      case 7:
        {

          if (forced_play(slotStatus.cross, slotStatus.circle, board))
            break;
          else if (fill_corner(slotStatus.cross, board))
            break;
          else fill_any(slotStatus.cross, board);
          break;
        }
      case 4:
      case 6:
      case 8:
        {
          if (forced_play(slotStatus.circle, slotStatus.cross, board))
            break;
          else if (fill_corner(slotStatus.circle, board))
            break;
          else fill_any(slotStatus.circle, board);
          break;

        }
  
    }
  
  }